The present invention relates to cloth-like polymeric films and products made therefrom, including polymeric backings for adhesive tapes.
Adhesive tapes containing a woven cloth backing are widely used, such as in conventional duct tapes sold to consumers and professionals. One of the advantages of using woven cloth as a tape backing is that it provides a combination of high tensile strength and low elongation to break in both the length or machine direction (xe2x80x9cMDxe2x80x9d) of the tape and in the width, or transverse direction (xe2x80x9cTDxe2x80x9d) of the tape. For many applications, this high strength and low elongation can be very important. Another advantage provided by tape backings is the relative ease of straight-line tearing along each of these two principle directions. Woven cloth backings also have drape and conformability characteristics which are desirable in many tape applications, and provide an appearance or xe2x80x9clookxe2x80x9d that is considered desirable in the marketplace for many kinds of adhesive tapes that have traditionally been made with woven cloth backings.
Woven cloth also has several disadvantages as an adhesive tape backing. In many applications, its woven structure must be saturated by the adhesive used in the tape, leading to the use of larger amounts of adhesive than for a film-backed tape. In some applications, a smooth or non-fibrillar top surface of the tape is desired, so the cloth backing must be overlaminated or coated. Also, hand-tearing cloth-backed tapes frequently results in loose fibers protruding from one or both torn edges. Finally, cloth backings are generally more expensive than polymer film backings.
For these reasons, a desire exists for a polymer film tape backing which combines, at low cost, the strength, elongation, tear, and conformability properties of woven fabric without the aforementioned disadvantages.
The present invention is directed to a polymeric film suitable for use as a backing material for tapes, as well as for use in other applications that require high tensile strength and straight-line tearing. The polymeric film has a plurality of perforations configured and arranged in a manner such that the film exhibits many of the properties of cloth materials. The configuration and arrangement of the perforations provide a film that can be readily torn in straight or substantially straight lines, yet has a sufficient tensile strength to be used in adhesive tape applications.
In specific implementations, the polymeric film comprises a composite film including a first polymeric film with a plurality of perforations, and a second polymeric film applied to the first polymeric film and substantially covering the perforations. The perforated film provides improved tearing properties, while the second film provides additional strength, durability, impermeability, or other desirable properties. In specific implementations an adhesive composition, such as a pressure sensitive adhesive, is applied to the film to make an adhesive tape.
The composite polymeric film is typically tearable by hand in at least one direction, and can be formed such that it is hand tearable in two perpendicular directions. The polymeric films of the invention have relatively low tear initiation energy and relatively high tear propagation energy compared to similar polymeric films that are not perforated in accordance with the invention. In addition, the perforated films of the invention allow tearing of the films in substantially straight lines compared to similar polymeric films that have not been perforated in accordance with the invention. The perforations allow such improved tear properties without excessively weakening the tensile strength of the film, and additionally provide a desirable reduction in the tensile elongation to break of the film.
Suitable polymers for the polymeric film include polyolefins, such as polypropylene for the first film and polyethylene for the second film. Various other polymers may be used. In certain implementations, multiple layers of films are combined to make a final composite film. In such implementations it is sometimes desirable to coextrude layers having high affinity for one another.
The perforations in the polymeric film are preferably non-circular and have a length at least 1.5 times their width, and typically at least 2 times their width. Although the perforations can show variations across the film, they typically each have a major axis and a minor axis. The major axis is a line along the length of the perforation, and the minor axis is a line along the width of the perforation. In one implementation, a line projected along the major axis of each perforation passes through an adjacent second perforation. In specific implementations a line projected along the major axis of each perforation passes through an adjacent perforation along or parallel to the minor axis of the adjacent perforation.
The invention is also directed to a method of making a composite polymeric film, the method including providing a first polymeric film comprising a first polymeric composition; perforating the first polymeric film; and applying a second film. The first film can be perforated with an infrared light source, a flame, or other method, and the film can be biaxially oriented prior to being perforated. In such implementations, perforation by radiation, flame, or other source of heat results in formation of a raised ridge along the edge of each perforation. This raised edge typically improves the tearing properties of the film.
The above summary of the present invention is not intended to describe each discussed embodiment of the present invention. This is the purpose of the figures, detailed description, and claims which follow.